Tomato Blight - Knowledgebase Question

I have been plagued with the tomato blight for years Done all the things they say to do, and I still have blight problems. I have been buying more plants and getting as much magnesium as possible to them. My question: When volunteer tomato plants come up from the wastes of the blighted tomatos that are buried in the compost pile, Why do they not have blight? I use and like the old ox heart and German beefsteak brands Is there a lesson here I should take notice and learn from?